You can choose your seats with Turkish Airlines once you have been provided with the airline's booking reference (also known as a locater or a PNR). You can find your flight reference by logging into My Booking on the Iglu website.
You will need to visit their website or open their mobile app. Click on the "Manage Booking" or "Check-in" link. You will then be prompted to enter your flight reference and your surname.
Once you have accessed your booking you will be able to choose your seats from the seat map.
Please be advised that when you are choosing your seats with Qatar Airways this is subject to their terms and conditions and fees may apply.